The most widely used dielectric fluids heretofore have been highly chlorinated hydrocarbons, such as the polychlorinated biphenyls (PCB's). While these are functionally satisfactory, they are highly objectionable from an environmental and ecological standpoint because of their high toxicity and extreme resistance to biodegradation. For this reason, acceptable substitutes have been actively sought.
Polyhalogenated diphenyl oxides are known to be useful as dielectric fluids, either alone (U.S. Pat. No. 2,022,634) or in admixture with alkyldiphenyl oxides (U.S. Pat. No. 2,169,995). Such fluids are no longer acceptable, however, because the polyhalo component is not sufficiently biodegradable.
Alkylated diphenyl oxides have been recommended for use as dielectric fluids (U.S. Pat. No. 2,170,809) but have never found wide use because of their poor electrical properties, especially their low dielectric constants.
Lower alkylchlorodiphenyl oxides are known and have been suggested for use as dielectric fluids (U.S. Pat. No. 2,170,989) but have not been accepted in the industry because of high volatility, low flash point and relatively poor electrical properties.